An ultrasonic transducer is known in the art and comprises a diaphragm secured to a voltage/oscillation transducer element which converts an electric voltage into a mechanical displacement which may be a change in the configuration or position or alternatively converts a pressure into an electric voltage.
FIG. 8 shows one exemplary ultrasonic transducer of the prior art. Specifically, there is provided a cup-shaped metallic diaphragm 1, to the internal surface of which is joined a voltage/oscillation transducer element 2 with its one electrode disposed in contact with the diaphragm 1. The element 2 comprises a piezoelectric element which becomes deformed in response to the application a voltage thereacross and which develops a voltage in response to the application of a pressure thereto. The other electrode of the element 2 is connected to one end of an electrical lead 3, the other end of which is connected to a terminal pin 4. The diaphragm 1 is connected to one end of another electrical lead 5, the other end of which is connected to a terminal pin 6.
A metal cap 7 is coated upon the bottom surface of a mount 8 of an insulating material, which is provided with a flange on its upper surface, to which a resilient member 9 in the form of an O-ring is joined. An O-ring 10 is fitted around the top end of the mount, and the assembly is fitted into the opening of the diaphragm 1, with its edge crimped into an annular configuration 11, thus securing the diaphragm 1 and the mount 8 together in an integral manner. The resilient member 9 and the O-ring 10 are effective to provide a water tight seal between the diaphragm 1 and the mount 8 to prevent the ingress of dust or liquid, typically water, while avoiding an interference with the oscillation of the diaphragm 1. When a voltage of ultrasonic frequency is applied across the terminal pins 4, 6, the element 2 expands and shrinks in the diametrical direction with the same frequency, whereby the diaphragm 1 oscillates, producing ultrasonic wave internally and externally of the diaphragm 1.
One example of the use of such ultrasonic transducer is illustrated in FIG. 9a. In this instance, an ultrasonic transducer is mounted on a rear bumper BR of a vehicle in order to detect the presence or absence of any obstacle adjacent to the rear portion of the vehicle and also to detect the distance to such obstacle. In the example shown, one transducer is used to transmit ultrasonic waves, and the reflected ultrasonic waves are detected by one or more additional transducers. In a different mode of use, the same transducer which transmits ultrasonic waves may be used to detect a reflected wave.
FIG. 9b shows a cross section of a base 12 on which transducers are mounted. In order to avoid any influence of the mechanical vibrations of a car body upon the operation of the transducers, transmitting and receiving ultrasonic transducers 1.sub.1 and 1.sub.2 are secured to the base 12 with resilient members 13.sub.1 and 13.sub.2, respectively, interposed therebetween.
As a result of the construction illustrated in FIG. 8 in which the dimension of the diaphragm 1 in a direction in which the ultrasonic wave is radiated or in a direction of the length of the pins 4, 6 is of a significant magnitude, and the mount 8 has an increased thickness because of the need to support an elastic seal structure comprising the resilient member 9 and the O-ring 10, added with the resilient members 13.sub.1, 13.sub.2 interposed, in combination with the projection of the terminal pins 4, 6 in the opposite direction from the direction of radiation of the ultrasonic wave, it will be seen that the sensor base 12 in which the transducers 1.sub.1, 1.sub.2 are received must have an increased thickness, resulting in an increased length of projection from the bumper BR. This tends to cause a collision of the base 12 with other things, causing the likelihood that the transducers 1.sub.1, 1.sub.2 may not be fully functional. In the manner of use illustrated in FIG. 9a, any article which may collide with the base 12 can be detected by the transducer prior to its collision, allowing the vehicle to be stopped. However, it is possible that articles may crash against or collide with the vehicle during the time it is at rest. In addition, such projection stands in the way of a car washing operation. Furthermore, it will be understood that the entire diaphragm 1 must be floating from the mount 8 and the base 12, and this causes the susceptibility that dusts or dirt may find its way into the space between the internal surface of opening formed in the base 12 and the external surface of the diaphragm 1, again causing a degraded functionability of the diaphragms 1.sub.1, 1.sub.2 or their inability to operate. Removal of dust or dirt is difficult to achieve, whereby the detection of an article may be imperfect.
A manner of use is also contemplated in which a transducer is stowed inside the bumper BR so that the latter may be abutted by articles without causing malfunctioning of the transducer. Such manner is illustrated in FIG. 10a which shows the resulting appearance, and also in FIG. 10b, which is an enlarged cross section thereof. In this manner of use, a sensor base 14 is horn-shaped in an attempt to provide a restricted range for the directivity of the ultrasonic wave and to preserve the intended use of the bumper BR, namely, removing any adjacent articles and preventing a collision of an article with the car body. A transducer is mounted in the smaller end of the horn. Again, the ingress of dust or dirt into the sensor base 14 is likely, and its removal is difficult, resulting in an imperfect detection of an article or articles.
On the other hand, a transducer of the kind described has an output of a low level. Accordingly, noises may be induced upon a transmission line which transmits an output signal from the transducer to a signal processor which is located at a suitable space in a vehicle. To provide a high signal-to-noise (S/N) ratio of a sensor signal which is applied to the signal processor, it is a conventional practice to dispose an amplifier adjacent to the transducer so that the output from the transducer may be amplified to a level at which the influences of induced noises are negligible before it is transmitted to the signal processor. However, noises induced in the transducer are also amplified. Accordingly, it is desirable that noises induced in the transducer be suppressed.